This application relates to the art of electric arc welding and more particularly to a method and system for determining the actual inductance of cables used in the welding installation.
Electric arc welding involves passing a current between an electrode and a workpiece in a manner to perform a desired arc welding process. To control the current during the welding process, the controller is set up to assume a certain impedance, including inductance and resistance, of the cables connecting the power supply with the electrode and workpiece at the welding station. Consequently, it is desirable to know the cable inductance and resistance to diagnose grounding problems and to improve the set up of the welding operation. Further, the actual inductance must be known to dynamically control the current flowing during the welding operation. In the field, the inductance of the cables used in the welding operation varies drastically. Welding processes are generally intolerant to poor circuit conditions that cause changes in the inductance of the cables. Further, some highly optimized welding processes require welding impedance within a narrow range of values. Consequently, the ability to measure and report or store inductance and/or resistance of the welding circuit is a very useful tool for application engineers in the field of electric arc welding. If the resistance or inductance values are outside specific ranges for acceptable operation, an application engineer can take corrective actions to adjust the welding circuit or factory installed welding programs, that are developed assuming certain cable impedance to obtain acceptable welding performance.
If the resistance of a cable is known to be beyond a certain limit, corrective action can be taken, such as replacing the cable or replacing moving parts in the grounding circuit. In a like manner, if the actual inductance of the welding circuit is known and is found to be too low, a length of cable can be coiled to achieve the desired inductance during set up before the process is implemented. In a like manner, if the actual circuit inductance is beyond a certain value, the welding cables can be shortened or gathered together to reduce the physical area enclosed by the welding cables to lower the inductance of the cables. To perform these corrective actions, the resistance and impedance of the welding cables must be determined on a real time basis or during set up. The value of such parameters may not be the same in prior set ups even if no discernable changes have been made to the installation.
One arrangement for measuring the actual inductance of the welding operation involves the use of leads at the power supply terminals. However, such leads have certain limitations. If one of the leads is connected to the electrode side of the installation, near the arc, and the other lead is attached to the power supply work connection, the impedance of the work lead itself is measured; however, this is not the impedance of the total circuit. In a like manner, connection of one lead to the work circuit near the arc and the other lead to the power supply electrode connection, the inductance measured is the electrode lead inductance. The use of these leads can identify certain problems; however, the actual inductance and resistance of the welding operation is not obtained. These systems have become necessary as welding control technology has advanced and become quite complex. Thus, accurate high speed voltage sensing has been used for the purpose of accurately controlling the dynamic arc voltage and measuring the transient events that occur in the arc. These systems involve a method for determining impedance of the welding circuit itself. The knowledge of the welding circuit inductance is used to optimize dynamic performance for the welding power supply. Thus, it is a substantial value to know the welding circuit impedance including inductance and resistance for use by the welding engineer. Also, this information is beneficial for machine set up, maintenance, troubleshooting, and diagnostics with respect to tooling and grounding problems. Consequently, systems have been developed in an effort to measure and record the inductance and resistance of the welding circuit. To obtain the inductance, a digital processing technique is employed. This technique involves increasing the current in a ramped straight line. By measuring the sensed voltage at a first current, awaiting a time dt and then recording a second sensed voltage at a second current, the di/dt can be calculated by the digital processor by the difference in current number over the time delay in digital format. The average voltage is determined by dividing the sum of the two sensed voltage by two. In a like manner, the average current is determined by dividing the sum of the two measured currents by two. Having these determinations made by the digital processor in the second phase of the calculation technique, the circuit inductance is obtained by digitally subtracting the average voltage from the product of the average current and circuit resistance to obtain a digital number or value divided by the digital representation of di/dt. Thus, a digital representation of circuit inductance is obtained. This technique produces a circuit resistance during the first stage and a circuit inductance during the second stage. However, these two stages are done successively while the electrode is shorted against the workpiece. The accuracy of the measurements is compromised by the variations which can occur from time to time during the testing procedure with the electrode shorted. The use of the two stage measuring operation to determine resistance and inductance of the welding operation still depends upon a measuring device, such as leads and the stability of the resulting values is less than optimum.
The digital processing procedure used for determining resistance and inductance of the welding process whereby the electrode is shorted and two measuring stages are performed is merely background to the present invention; however, it is not necessary prior art to the present invention. However, such prior two stage digital processing procedure has been improved by the present invention consisting of a method for determining the inductance of the network connecting the electrical power supply to an electric arc welding station. The network is normally a cable and gun, or a pair of cables, between the power supply and the electrode contact tip and workpiece at which the welding process is implemented. In accordance with the invention, the electrode is shorted to the workpiece so that the arc voltage is zero. Then, a series of waveforms are created. Each of these waveforms includes a first state operated by the power supply at a constant current of a selected value. In practice, this value is at least 300 amperes, which is maintained for approximately 10 ms. The waveform then includes a second state wherein the power supply is operated at a low value of open circuit voltage so that the current through the shorted circuit decays from the selected value along a decay path having a curve determined by the value of the impedance of the circuit. In practice, the open circuit voltage in the second state is zero; however, it could be at a value less than about 2.0 volts. While the electrode is shorted, many of these waveforms are sampled in succession. During each of the waveforms, the first state is digitally processed to obtain the average current and the average resistance. This operation involves taking several samples of instantaneous voltage and instantaneous current. These values are converted to digital numbers. The average resistance is the summation of the arc voltage at a sampling time divided by the instantaneous current at a sampling time. The summation is then divided by the number of samples to obtain the resistance of the cable or connecting network. This digital process provides the average resistance for the network. At the same time, during the first state the sampled instantaneous current is summed and divided by the sampling number to obtain the average current for the power supply during the constant current first state of the measuring procedure.
After the first state, the waveform transitions into the second state. A time portion for the current decay path is selected. This portion starts after the current delay has been initiated by shifting the power supply to zero open circuit voltage. This can be done by shifting the power supply to zero current. Shifting to a low value is the general or broad concept. Zero output is the best to avoid noise. The current decay is concluded before the power supply is shifted back to the constant current mode. In practice, this timed portion is about 4.0 ms. During this time portion, a substantial number of instantaneous current samples are taken. Each of the samples occurs about each 0.10 ms. Since the time portion is approximately 4.0 ms, in this second sampling procedure, approximately 40 current samples are recorded. In the first state, about 10 current and voltage samples are taken over approximately 1.0 ms. Consequently, the invention involves interrogating the actual waveform approximately ten times during the constant current or first state and approximately 40 times during the zero open circuit voltage or second state. Both of these states are implemented in approximately 30 ms. Thereafter, the states are repeated and the digital process of the samples is also repeated. During the first state, the average current and average resistance is obtained very accurately due to the large number of samples during a very short period of time after the current has stabilized. In a like manner, the current decay rate during the second state is accurately measured due to the sampling process when a large number of current samples are processed. The instantaneous current is sampled during the time portion in the second state. By using the instantaneous current and the average current obtained during the first state, together with the average resistance from the first state, the instantaneous inductance is determined by digital processing of these values in the second state. Then, the microprocessor or other digital processor sums the instantaneous inductance values calculated 40 times during the second state. By dividing the resulting accumulated inductance values by the sampling number performed in the second state, the actual inductance in the connection network is obtained for use in controlling the welding process and in setting up the welding operation.
In accordance with an aspect of the invention, the time portion of the decay curve in the second state is delayed a short time until the current has decayed to a certain value. Thereafter, the 4.0 time portion allows implementation of the 40 sample measurements to obtain 40 instantaneous inductance numbers along a stable portion of the decay path.
The present invention is implemented digitally; however, the waveforms are displayed for viewing by the welding engineer. In essence, the present invention involves the creation of waveforms which may be displayed and then using these waveforms to first determine certain actual parameters of the welding operation and then using these parameters together with an analysis of the decaying current curve to determine the actual inductance of the circuit. The present invention is an improvement over a system wherein the first state is used to measure a parameter, such as resistance, and then the next state is a variable current state which drives the current in a ramped fashion to determine the inductance. Such ramping of the current is not as accurate and is not as stable as the current decay used in the present invention. Nevertheless, the invention is broad enough to include a method where the instantaneous inductance is determined in a time based second state using the average resistance with the instantaneous measurement of the current periodically along the decay curve of the second state to obtain the inductance of the network.
In accordance with the preferred embodiment of the invention, the instantaneous inductance is obtained by multiplying the negative of the average resistance obtained during the first state by the time and divide this product by the natural log function of the instantaneous current divided by the average current. This instantaneous inductance is then averaged over the sample numbers. In this manner, the digital processor employs the natural log function in combination with a large number of samples to stabilize the digital process. Consequently, an accurate reading of the total inductance of the actual circuit and the actual resistance is obtained for use in controlling the welding process or setting up the welding installation.
In the preferred embodiment of the invention described above, the time relationship during the current decay curve in the second state of the waveform is read by several samples of the instantaneous current divided by the average current during the first state whereby the natural log function of this quotient is used in reading the decay curve. It has been found that this technique produces the best stability due to the smaller standard deviation of the sample readings taken at approximately ten readings per millisecond. In accordance with another embodiment of the invention, the current decay curve is analyzed, or read, as a di/dt determination wherein the prior current sample is compared to the next sample. It has been found that this second embodiment does provide as accurate determination of the inductance of the network including a cable or cables as compared to the preferred technique; however, the sample readings over the set sampling time have been found to vary from sample to sample when consistency would be expected. Thus, these samples must be averaged to obtain a relatively accurate total system inductance. The first embodiment has been found to have a less standard deviation than the second embodiment even though the second embodiment does employ the concept of the invention wherein a waveform is created having a constant current state followed by a low voltage or zero open circuit voltage and where several samples are taken during the first state to measure average resistance and average current of the network and many samples during the second state reads the decay curve. This is the broad aspect of the invention which can be employed in either of the two embodiments set forth herein or other embodiments using this same inventive digital sampling and processing techniques.
The primary object of the present invention is the provision of a method of obtaining the precise accurate inductive reactance of the connecting network, such as a cable or cables, between the power supply and the welding station of an electric arc welding installation. This invention involves the formation of a waveform and the sampling of the waveform at precise locations to provide sampling numbers which are processed digitally to provide the desired inductance and/or resistance values.
Another object of the present invention is the provision of a method, as defined above, which method can be implemented using a standard digital processor available in electric arc welding controls by merely implementing the digital processing technique constituting the present invention.
A further object of the present invention is the provision of a method for determining the impedance of the welding cables in a welding operation by creating a series of waveforms having a first state which is constant current and the second state which is a low or zero open circuit voltage wherein each state is processed with the electrode shorted to the workpiece. In using this method, a current decay curve is created and is read to provide the inductance of the network by using the actual resistance of the network.
Still a further object of the present invention is the provision of a method, as defined above, which method provides an accurate determination of the network inductance without requiring the concept of increasing voltage and periodically reading the current to measure di/dt.
Yet another object of the present invention is the provision of a method, as defined above, which method can be implemented by many embodiments involving the reading of the decay curve in the second state of each waveform with many instantaneous current samples to provide measurement reliability.
Another object of the present invention is the provision of a method of controlling an electric arc welder utilizing the inductance determined by using the present invention and/or the resistance obtained by using the present invention to modify the closed loop feedback of the welder controller.
A further object of the invention is the provision of a method, as defined above, which method uses rapid sampling during a stable segment of a constant current mode and a stable portion of the decay curve when the power supply shifts to a zero open circuit voltage mode.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings.